Boiler loses pressure after heat comes on

[Mander]

I have a pretty old combi boiler (glow-worm 24ci), it's at least as old as 2007 when I bought my house. I've had to have a repair done about every two years or so as it will lose pressure repeatedly. This usually happens after I've turned the thermostat down while I'm on holiday.

In the past I had a British Gas boiler service contract (since cancelled because it was too expensive), and they have been able to fix it by removing and cleaning a part (expansion vessel perhaps?) but they always say that a replacement part is no longer available. I just came back from holiday and lo and behold I'm having the same problem.

If I top up the pressure and then just use hot water it keeps working, at least for most of the day. However, if the heat comes on the pressure will drop again after it cycles off. I don't know anything about boilers but I'm generally OK with DIY things (with appropriate instructions). What do I need to check or look at in order to try and diagnose the problem? I was wondering if I could identify the offending part and look for an alternative supplier before calling in someone to service it.

 

[undertrained]

When your heating is on does the pressure rise at all?

 

[Mander]

Hmm, it's stupid but I don't know how to tell, as the digital display changes to show the water temperatue. I will have to figure out how to check that.

 

[Mander]

On further observation it would appear that the pressure goes up when the heat is on, as the boiler will run until the appropriate temperature is reached and then it will keep working for hot water etc. until the next time the heat tries to kick in, at which point it loses pressure and goes off again.

 

[ianmcd]

your expansion vessel needs re-charged/replaced also check the communication hose/pipe for blockages

 

[Mander]

You were right, that's exactly what the repair guy ended up doing (recharged the expansion vessel). He only actually had to clean some gunk out of it and possibly the PRV, which is what I think I've had to have done in the past.

He also said the whole system needs to be flushed. Somehow I have had the impression that flushing a system isn't actually necessary, but I don't know where that came from. No idea how long ago this system has been flushed, if it ever has, but it was at least 12 years ago. The repair guy was talking about taking all the radiators off the wall, cleaning them out, and then cleaning the rest of the system. Is this different from the "power flush" that British Gas has tried to sell me in the past? Is it something I should have done? He quoted me 250 pounds (including a discount since I already hired him to do the other work).

 

[ianmcd]

That is a really good price go for that , the powerflush quote is a con to be honest, a powerflush sometimes works but only if carried out correctly and BG wont do it properly